Structural Characterization and Surface Modification of Evaporated

Advanced Materials Research Laboratory, Matsushita Research Institute Tokyo, Inc.,. 3-10-1 Higashimita, Tama-ku, Kawasaki 214, Japan. Received June 10...
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Langmuir 1996, 12, 5451-5457

5451

Structural Characterization and Surface Modification of Evaporated Thin Films of 5,5′′′-Bis(aminomethyl)-2,2′:5′,2′′:5′′,2′′′-quaterthiophene and Its Dihydrochloride Hitoshi Muguruma,*,† Takashi Saito, Atsunori Hiratsuka, and Isao Karube Research Center for Advanced Science and Technology, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153, Japan

Shu Hotta Advanced Materials Research Laboratory, Matsushita Research Institute Tokyo, Inc., 3-10-1 Higashimita, Tama-ku, Kawasaki 214, Japan Received June 10, 1996X Thin films of 5,5′′′-bis(aminomethyl)-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (1) and its dihydrochloride (2), both of which possess active amino groups at both the molecular terminals, have been fabricated by direct evaporation deposition. IR and XPS analyses have revealed that these thin films are deposited without causing chemical degradation during the course of deposition. Of these, the thin film of 1 is characterized by the molecular layered structure that can be noticed for the oligothiophene compounds widely, and in this structure the molecular long axis of 1 is nearly vertical to the substrate plane. On the other hand, the film of 2 turned out to be amorphous. The structure of 1 immediately means the presence of a densely packed array of the amino groups at the thin film surface. Making use of this structure, we tried surface modification of the thin film of 1 with an aldehyde. The resulting film was analyzed by XPS spectroscopy, and as a consequence, we confirmed that the surface modification is successfully achieved. Furthermore, we attempted immobilization of glucose oxidase (GOD) onto the thin film of 1 and examined its catalytic activity.

Introduction Oligothiophenes are currently attracting increasing attention as model compounds of polythiophenes1-4 and as a new class of organic semiconducting materials.5-11 These oligothiophenes show interesting electronic properties, e.g. high mobility, which has encouraged many researchers to make thin-film electronic devices such as field-effect transistors8-10 and light-emitting diodes.11,12 From the point of view of the morphology, the oligothiophenes are also characterized by a well-developed molecular layered structure both in single crystals and thin films.5,6 Owing to this structure, the molecular long axis of the oligothiophenes in thin films is nearly vertical or slightly oblique against the substrate plane; molecules tend to stand upright against the substrate with increasing polymerization degrees of the compounds. This structure partly results from the strong π-π interaction between the well-extended molecular backbones. X-ray diffraction analysis of R,ω-dialkyl-substituted oligothiophenes shows † Present address: Department of Chemical and Biochemical Engineering, Rutgers University, Piscataway, NJ 08855-0909. X Abstract published in Advance ACS Abstracts, October 1, 1996.

(1) Zotti, G.; Schiavon, G.; Berlin, A.; Pagani, G. Chem. Mater. 1993, 5, 620. (2) Horowitz, G.; Yassar, A.; von Bardeleben, H. J. Synth. Met. 1994, 62, 245. (3) (a) Ba¨uerle, P.; Segelbacher, U.; Maier, A.; Mehring, M. J. Am. Chem. Soc. 1993, 115, 10217. (b) Ba¨uerle, P. Adv. Mater. 1992, 4, 102. (4) Guay, J.; Kasai, P.; Diaz, A.; Wu, R.; Tour, J. M.; Dao, L. H. Chem. Mater. 1992, 4, 1097. (5) Hotta, S.; Waragai, K. J. Mater. Chem. 1991, 1, 835. (6) Hotta, S.; Waragai, K. Adv. Mater. 1993, 5, 896. (7) Hotta, S.; Waragai, K. J. Phys. Chem. 1993, 97, 7427. (8) Akimichi, H.; Waragai, K.; Hotta, S.; Kano, H.; Sakaki, H. Appl. Phys. Lett. 1991, 58, 1500. (9) (a) Garnier, F.; Yassar, A.; Hajlaoui, R.; Horowitz, G.; Deloffre, F.; Servet, B.; Ries, S.; Alnot, P. J. Am. Chem. Soc. 1993, 115, 8716. (b) Garnier, F.; Hajlaoui, R.; Yassar, A.; Srivastava, P. Science 1994, 265, 1684. (10) Dodabalapur, L.; Torsi, L.; Katz, E. Science 1995, 268, 270. (11) Geiger, F.; Stolt, M.; Schweizer, H.; Ba¨uerle, P.; Umbach, E. Adv. Mater. 1993, 5, 922. (12) Uchiyama, K.; Akimichi, H.; Hotta, S.; Noge, H.; Sakaki, H. Synth. Met. 1994, 57, 63.

S0743-7463(96)00570-7 CCC: $12.00

that the alkyl-substitution causes a better-developed molecular layered structure than that for the nonsubstituted compounds.5,9a,12 This means that interaction between the substituent groups is thought to play an important role as well in determining the structure and morphology of the thin film. Making use of these structural characteristics, various oligothiophene composites were developed. Examples include Langmuir-Blodgett films where the oligothiophenes are suitably assembled with a saturated fatty acid.13,14 If the oligothiophenes are allowed to have chemically active (convertible) groups as substituents, these oligothiophenes also produce useful composites. A good illustration for this is a multilayered structure involving zirconium that results from alternate selective chemisorption of the zirconium ions and quaterthiophenediphosphonic acid on top of each other.15 The latter example implies that if the molecular layered structure persists in thin films consisting of compounds that have the chemically active groups at both the molecular terminals, their surface will be covered with a densely packed array of these active groups. Another implication of this example is that if a suitable reagent is chosen, selective chemisorption most likely takes place effectively onto the surface. In this context it is quite intriguing to study more intensively the structural characterization and surface modification of the thin films of oligothiophenes suitably modified with the chemically active groups. From the point of view of practical purposes, in turn, evaporation deposition is a quite versatile method of thin film fabrication for materials both organic and inorganic. As far as the organic compounds with chemically active (13) Paloheimo, J.; Kuivalainen, P.; Stubb, H.; Vuorimaa, E.; YliLahti, P. Appl. Phys. Lett. 1990, 56, 1157. (14) Nakahara, H.; Nakayama, J.; Hoshino, M.; Fukuda, T. Thin Solid Films 1988, 160, 87. (15) (a) Katz, H. E.; Schilling, M. L.; Chidsey, C. E. D.; Putvinski, T. M.; Hutton, R. S. Chem. Mater. 1991, 3, 669. (b) Katz, H. E. Chem. Mater. 1994, 6, 2227.

© 1996 American Chemical Society

5452 Langmuir, Vol. 12, No. 22, 1996 Scheme 1

groups are concerned, however, their direct evaporation deposition is virtually limited to the case of evaporation polymerization.16-18 This is probably because such compounds are thought to be easily subject to chemical alteration or degradation during the course of deposition. In the case of the evaporation polymerization, the compounds are rather designed to efficiently cause polymerization during and/or after evaporation. Therefore, the evaporation processibility of the compounds having the chemically active groups is worthy of being pursued more widely. To address the above-mentioned subjects, we chose 5,5′′′bis(aminomethyl)-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (1) and its dihydrochloride (2) as model compounds both of which possess active amino groups at both the molecular terminals (see Scheme 1 for chemical structures). As an initial attempt, we have made thin films of these compounds via the evaporation deposition and characterized them by spectroscopic and X-ray diffraction methods. The surface modification has been carried out on the thin films of 1 using an aldehyde compound. Taking a further step forward, we immobilized glucose oxidase (GOD) onto the thin film of 1 and examined its catalytic activity. Experimental Section Materials. The syntheses and purification methods of compounds 1 and 2 can be seen elsewhere.19 These compounds were vacuum-deposited on KBr disks (for IR spectroscopy) or (16) Iijima, M.; Takahashi, Y. Macromolecules 1989, 22, 2944. (17) Yudasaka, M.; Karl, N. Thin Solid Films 1990, 187, 165. (18) Yoshimura, T.; Tatsuura, S.; Sotoyama, W.; Matsuura, A.; Hayano, T. Appl. Phys. Lett. 1992, 60, 268. (19) Muguruma, H.; Saito, T.; Sasaki, S.; Hotta, S.; Karube, I. J. Heterocycl. Chem. 1996, 33, 173.

Muguruma et al. quartz glass slides [for UV-vis, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS)] at a temperature of ca. 180 °C under a pressure of 7 × 10-4 Pa. The thickness of the deposit was regulated suitably depending on the experiments (∼500 nm for the IR, X-ray diffraction, and XPS experiments and ∼100 nm for the UV-vis experiment). The as-deposited films of 1 and 2 will be henceforth referred to as 1a and 2a, respectively, to designate the thin film explicitly. The methods of the surface modification of the thin films are summarized in Scheme 1. For that purpose, 1a was dipped in a 0.1 M pentafluorobenzaldehyde (PFB) solution of hexane for 1 h at room temperature under an ambient environment. The sample was then sufficiently washed with hexane and further soaked in hexane for 6 h to ensure thorough removal of surplus PFB at and near the film surface (sample 1b). Note that PFB is a well-known reagent to derive a Schiff base (see Scheme 1); the detailed experimental processes for this can be seen in the literature.20 To immobilize an enzyme of GOD onto the thin film, we prepared a glass substrate coated with plasma-polymerized hexamethyldisiloxane,21,22 which causes good adhesion between the glass substrate and 1a. Onto this substrate 1 was vacuumdeposited and successively exposed to glutaraldehyde (GA) vapor for 30 min. Subsequently, a GOD (Sigma Co.) solution (5 mg/ mL) with phosphate buffer (20 mM, pH 5.6) was dropped onto 1a. After ensuring the GOD immobilization by a 30-min soak, the resulting thin film was rinsed well with distilled water (sample 1c). The thin films both as-deposited and surfacemodified were analyzed by XPS to examine whether the surface modification had progressed as desired. Measurements. The thin films were analyzed by the spectroscopic and θ-2θ X-ray diffraction methods. A detailed description of the measurement and apparatus of X-ray diffraction together with the IR and UV-vis spectroscopy can be found in already published literature.5 Doping experiments were carried out following the previously described manner.5 The conductivities both before and after the doping were measured by a standard two-probe method about the thin films which were vacuumdeposited onto a pair of gold electrodes evaporated on a glass substrate. The XPS apparatus was a Shimadzu ESCA 750 (Mg KR, 1253.6 eV). The X-ray source was operated at 8 kV and 30 mA, and the pressure during the measurement was N+H‚‚‚Br- > N+H‚‚‚N.32 The appreciably strong peak at 1595 cm-1 is attributable to the NH in-plane deformation, δ(NH3+).25 The ring stretching modes ν(ring) can be seen at 1514, 1460, and 1441 cm-1. The γ(CH) mode for 2a is spilt into two at 791 and 777 cm-1; the former mode is more intense than the latter. This observation is opposed to that for the as-synthesized material of 2. Thus, on the basis of the close inspection of the IR spectra of 1a and 2a and the strong resemblance of their spectral profiles to those of the as-synthesized materials, we conclude that no chemical alteration or degradation of these compounds has taken place and that their chemical structures remain intact. This is contrary to our first anticipation, especially with compound 2; at first glance the chemical structure of 2 made us assume that hydrochloric acid is easily extricated and pumped out during the course of vacuum deposition. The UV-vis spectra of 1a and 2a show features similar to those of nonsubstituted quaterthiophenes33 and alkyl(29) Badger, R. M.; Waldron, R. D. J. Chem. Phys. 1957, 26, 255. (30) Novak, A. Spectrochim. Acta 1960, 16, 1001. (31) Bellanato, J. Spectrochim. Acta 1960, 16, 1344. (32) Sigu¨enza, C.; Galera, P.; Otero-Aenlle, E.; Gonza´lez-Dı´az, P. F. Spectrochim. Acta 1981, 37, 459. (33) Fichou, D.; Horowitz, G.; Xu, B.; Garnier, F. Synth. Met. 1992, 48, 167.

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Figure 2. θ-2θ X-ray diffraction patterns of (a) 1 and (b) 2. Τhe bottom is for the evaporated films 1a and 2a. The top is the powder pattern of the as-synthesized materials for comparison. The peaks marked with asterisks in (a) were not due to the higher order reflections of the primary diffraction.

substituted ones;5 the π-π* transition occurs at the visnear UV region as a broad and dominant band. Yet the peak locations of 1a and 2a observed at 375 and 367 nm, respectively, are significantly red-shifted relative to those of a related quaterthiophene compound 5,5′′′-dimethyl2,2′:5′,2′′:5′′,2′′′-quaterthiophene (DMQtT).7 The conductivities of 1a before and after being doped with NOBF4 were 10-10 and 10-3 S/cm, respectively. The corresponding values of 2a were 10-8 and 10-7 S/cm. These optical and conductivity data reflect the semiconducting properties that are defined by the electronic structure characteristic of the quaterthiophene backbone. This is not surprising because the amino groups are linked to thiophene rings through the medium of electronically inert methylene groups. X-ray Diffraction. Parts a and b of Figure 2 depict the θ-2θ X-ray diffraction patterns of 1a and 2a, respectively. The powder X-ray patterns of the assynthesized materials are shown for comparison. Film 1a exhibits a set of sharply resolved diffraction peaks that are due to a primary diffraction spacing of 17.5 Å and its higher order reflections. A broad peak around 4 Å (ca. 20-30°) observed for the diffraction diagrams of the thin film samples arises from the quartz substrate on which the oligothiophene was deposited. The reflections up to the eightth order can be noticed for 1a, apart from the peaks marked with asterisks (Figure 2a), which were not assigned to the higher order reflections of the primary diffraction. These results obviously demonstrate that 1a comprises a well-defined molecular layered structure. The presence of the hydrogen bonding between the amino groups, which is evidenced in the IR spectrum, is probably responsible for this structure. The layered structure is characteristic of various oligothiophene compounds more generally.5 As an example, a thin film of DMQtT exhibits the corresponding primary diffraction spacing of 18.3 Å.5 Note that 1 is similar to DMQtT in molecular size because 1 has the

Muguruma et al.

Figure 3. XPS full spectra of (a) 1a and (b) 2a.

aminomethyl groups at both the molecular terminals instead of the methyl groups for DMQtT. Since 1a gives a bit smaller primary diffraction spacing than the DMQtT thin film, we infer that the tilting angle of the 1 molecules against the normal of the substrate is somewhat larger than that for DMQtT (∼26°) but not very different. This immediately means that the amino groups are densely present at the surface of 1a, since these amino groups are located at both the molecular terminals. The assynthesized material of 1 shows the powder X-ray pattern nearly identical to that of 1a except for a few more peaks that are not ascribed to the higher order reflections of the primary one. As for the thin film 2a, on the other hand, we observed only one weak peak of 3.85 Å, indicating that the film is amorphous and lacks the regular layered structure. The peak intensity is about one order of magnitude smaller than that of 1a. This clearly shows how the chemical modification (by hydrochloric acid) affects the overall morphology of the material. Compared with that of 2a, the powder X-ray pattern of the as-synthesized compound 2 exhibits several distinct peaks at 5.69, 5.61, 4.02, 3.86, 3.39, 3.23, 2.81, and 2.42 Å, indicating that the material is partially-crystalline. The layered structure could not be observed either. Special care, however, should be taken about whether chlorine anions in the evaporated thin films are situated in a manner exactly the same as that for the as-synthesized material. Once the materials including the chemically active groups are found to assume the molecular layered structure, as in the case of 1a, it will be of great importance to study subsequent surface modification of the resulting thin film using a suitable chemical reagent. XPS Spectroscopy. Parts a and b of Figure 3 show the full spectra of XPS of 1a and 2a, respectively. Peak positions and their assignments are collected in Table 1. Elemental ratios determined from the peak areas are listed in Table 2. The spectra of important components for 1a and 2a are depicted more precisely in parts a and b of

Bis(aminomethyl)-2,2′:5′,2′′:5′′,2′′′-quaterthiophene

Langmuir, Vol. 12, No. 22, 1996 5455

Figure 4. More precise features of the XPS spectra of the important components for (a) 1a, (b) 2a, and (c) 1b. Table 1. XPS Peak Positions (eV) and Their Assignmentsa cmpd 1a 2a 1b

C 1s

N 1s

S 2p3/2

S 2p1/2

F 1s

Cl 2p1/2

Cl 2p3/2

284.8 286.3 399.3 164.2 165.4 284.9 286.6 401.5 164.2 165.5 197.9 199.5 284.8 287.9 401.3 164.2 165.6 689.2

a The peak location of the S 2p 3/2 line is defined as 164.2 eV (see the text).

Table 2. Elemental Ratios Determined from XPS N/C

S/C

cmpd

expt

theory

expt

theory

1a 2a 1b

0.083 0.087 0.041

0.111a 0.111b

0.200 0.196 0.090

0.222a 0.222b

a

F/C expt

Cl/C expt

theory

0.085

0.111b

0.305

Stoichiometric ratio for 1. b Stoichiometric ratio for 2.

Figure 4, respectively. The S 2p band of 1a can be resolved into two lines related to species with different spins of S 2p3/2 (at 164.2 eV) and S 2p1/2 (165.4 eV). The peak intensity ratio of the former to the latter is 2:1, in agreement with theory.24,34 The peak locations of other elements were calibrated against the S 2p3/2 line, whose peak location is defined as 164.2 eV,34 because this line (the full width at half maximum (fwhm) ) 1.2-1.3 eV) is sharper than any others. The C 1s and N 1s lines of 1a are observed at 284.8 and 399.3 eV, respectively, also with a relatively sharp feature (fwhm ) 1.5 eV). The peak position of the latter line is closely related to that of amino groups already reported (399.2 eV)34 and, hence, is assigned as such. The thin film 2a shows the C 1s band (284.9 eV) with a peak position and a profile similar to those for 1a. The (34) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. In Handbook of X-ray Photoelectron Spectroscopy; Muilenberg, G. E., Ed.; Perkin-Elmer Corp.: MN, 1979; pp 1-57.

N 1s line of 2a, however, is shifted toward the higher energy side by 2.2 eV relative to that for 1a and located at 401.5 eV, being assigned to protonated nitrogen, NH3+.34 The Cl 2p band can be resolved into two lines at 197.9 and 199.5 eV due to the two different spin species of Cl 2p3/2 and Cl 2p1/2, respectively. The peak intensity ratio of the former to the latter is 2:1, again in agreement with the theory.24,34 The peak at 532.0 eV for 1a and 2a is attributed to the O 1s line of oxidized carbon. The occurrence of this line is concomitant with a weak shoulder around 286-287 eV of the C 1s band which is resolved from the convolution analysis. This shoulder presumably results from oxidized carbon such as CdO and CsOH.34 On the other hand, no oxidized species of nitrogen or sulfur could be detected for 1a nor for 2a; if the amino groups were changed into NO or NO2 groups by oxidation, the N 1s peak would be observed around 406 eV.34 If, moreover, the sulfur was changed to SO3 or SO2, the S 2p peak would be observed around 168 eV.34 However, this was not the case with our experiments. In spite of the presence of the oxidized carbons, this would not mean chemical degradation during the evaporation deposition. Such contamination is rather ascribable to 1a and 2a being oxidized by air even during their careful handling. The XPS spectra of 1b in Figure 4c clearly demonstrate that the N 1s line is shifted toward the higher energy region relative to that of 1a, even though the convolution analysis of that band is indicative of the presence of a trace of unmodified amino groups (at 399.5 eV). The major band of the N 1s peak at 401.3 eV is due to azomethine groups NdCH.34 A very intense line of the F 1s band at 689.2 eV (see Imax in the figure) clearly shows itself in the spectra. Concomitantly, the C 1s band comprises two components, the higher energy line of which is attributed to the carbon atoms bound to fluorine. On the other hand, the S 2p peak of 1b is almost unaffected both in the peak

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position and profile compared to the corresponding peak of 1a. These XPS results indicate that the surface modification using PFB has been achieved satisfactorily. To do quantitative evaluation, we start with the following consideration. When the atoms to be detected are homogeneously distributed, the photoelectron signal intensity occurring from those atoms can be expressed as35,36

Ix ) I0 exp[-x/(λ sin θ)]

(1)

where Ix is the photoelectron signal intensity from a portion of the thin film of depth x from the film surface, I0 is the signal intensity from the surface, λ is a mean free pass of a photoelectron, and θ is the photoelectron take-off angle defined as that between the detector axis and the film surface. Therefore, the ratio (R) of signals arising from portions whose depth from the surface is d or smaller to those arising from the entire film of thickness t0 is given by

Strictly speaking, this opposes the premise of eq 1 and may well lead to deviation of the observed N/C and Cl/C ratios from the stoichiometric ones. In our case, such deviation up to 25% was noticed (Table 2). In order to examine whether the surface modification of 1a has progressed as desired (see Scheme 1), we estimate the N/C and F/C ratios both before and after the surface modification. Suppose before the surface modification there are nN nitrogen atoms and nC carbon atoms per unit volume. If all the amino groups have been converted into azomethine groups by the PFB modification, it follows that each nitrogen atom produces seven carbon atoms and five fluorine atoms. As a consequence, after completing the surface modification the elemental ratios are approximated as37

NN/NC ) (nN/nC)/[1 + 7(nN/nC)]

(5)

∫0dexp[-x/(λ sin θ)] dx/∫0t exp[-x/(λ sin θ)] dx

NF/NC ) 5(nN/nC)/[1 + 7(nN/nC)]

(6)

) [1 - exp(-d/(λ sin θ))]/[1 - exp(-t0/(λ sin θ))] (2)

where NN, NC, and NF denote the numbers of nitrogen, carbon, and fluorine atoms per unit volume after the PFB modification, respectively. The volume change ensuing from the PFB modification has been taken into account. If these atoms were distributed homogeneously, we should be able to evaluate the atomic ratios directly from the XPS observations. This is, however, not the case with the N and F atoms, and so this might again make the estimation based upon eqs 5 and 6 somewhat incorrect. Yet the observed F/C (N/C) ratio of 0.305 (0.041) agrees with the F/C (N/C) ratio 0.262 (0.053) estimated on the basis of eqs 5 and 6 within about 20% (Table 2). This observed F/C ratio is quite high compared with the previously reported ones,20 which is a consequence of the presence of the amino groups densely packed at the surface. Nevertheless, we cannot tell from the XPS results alone how deep the surface modification has reached. Instead of answering this question directly, we point out that the X-ray diffraction peaks of 1b decreased their intensity by ∼20% in relation to those of 1a, while retaining the related peak locations and profiles. This is indicative of the persistence of the well-defined molecular layered structure and implies that the chemical modification using PFB takes place in the outermost parts of 1a at and very close to the film surface. Electrochemical Data. Since PFB is shown to be incorporated in the thin film 1a in a well-ordered form, it should be worthwhile to further study the surface modification using other useful materials. To this end, we tried immobilization of GOD (formula weight: 186 00038 ), which has a lot larger molecular weight than PFB (formula weight: 196), through the medium of glutaraldehyde as a cross-linking reagent (see Scheme 1, 1c). Figure 5 shows the cyclic voltammograms of 1c (a) without glucose and with (b) 40 mM and (c) 80 mM glucose. The current response is attributed to electrons generated by the oxidation of hydrogen peroxide (see eq 8 below), and so the amount of increasing current should be proportional to the concentration of glucose. Note that GOD catalyzes the following reaction:38

R)

0

In the meantime, λ (in nm) of an organic compound may be expressed as36

λ ) 49/FE2 + 0.11E0.5/F

(3)

where F is the bulk density of the organic compounds in grams per cubic centimeter and E is the electron kinetic energy in electronvolts. By replacing E in eq 3 with the experimental values and assuming F ) 1.44 g/cm3, which is taken from DMQtT,5 the λ is estimated at ca. 24, 25, 22, 25, and 18 Å for the photoelectrons emitted from the C 1s, S 2p, N 1s, Cl 2p, and F 1s levels, respectively. Furthermore, replacing eq 2 with θ ) 90° and t0 ) 500 (nm) in the present studies and considering exp(-t0/λ) , 1 for all the above photoelectrons, we obtain

R ≈ 1 - exp(-d/λ)

(4)

Taking d ) 3λ, R is roughly 0.95. In other words 95% signals are supposed to come from the top layer of thickness about 55-75 Å. Since the monolayer thickness of 1a is 17.5 Å, as confirmed from the X-ray diffraction study, we presume that the XPS signals mostly come out of several monolayers at and near the surface of the film. Within this region the C atoms along with the S atoms can be regarded as being distributed homogeneously, and the relative sensitivity factors of the C 1s and S 2p lines (see Experimental Section) are reliable. In fact, the agreement of the observed S/C ratio both for 1a and 2a with that expected from stoichiometric calculation is appreciably satisfactory (within an error of ca. 10%). This is not true, however, of the N atoms. Note that within a molecule 1 the two N atoms are located at the molecular terminals, separated from each other by four thiophene rings and two methylene groups (see chemical structures in Scheme 1). In a pair of adjacent molecules of 1 situated in the depth direction within the film, however, these N atoms are in close contact. Although 2a is amorphous, the N and Cl atoms in 2a would not be distributed homogeneously either. (35) Cammarata, V.; Atanasoska, L.; Miller, L. L.; Kolaskie, C. J.; Stallman, B. J. Langmuir 1992, 8, 876. (36) Roberts, R. F.; Allara, D. L.; Pryde, C. A.; Buchanan, D. N. E.; Hobbins, N. D. Surf. Int. Anal. 1980, 2, 5.

and

β-D-glucose + O2 f D-glucono-1,5-lactone + H2O2 (7) (37) Muguruma, H.; Karube, I.; Saito, M. Chem. Lett. 1996, 283. (38) Seikagakujiten, 2nd ed.; Tokyo-Kagakudojin: Tokyo, 1990; p 392 (in Japanese).

Bis(aminomethyl)-2,2′:5′,2′′:5′′,2′′′-quaterthiophene

Langmuir, Vol. 12, No. 22, 1996 5457

Figure 5. Cyclic voltammograms of 1c (1 × 2 cm2) recorded in 10 mL of 20 mM phosphate buffer (pH 5.6) (a) without glucose and with (b) 40 mM and (c) 80 mM glucose.

The hydrogen peroxide produced further reacts with the Pt working electrode as follows:39

H2O2 f 2H+ + O2 + 2e-

(8)

The results shown in Figure 5 thus demonstrate that the immobilized GOD on 1c exhibits the normal enzymatic activity and exemplify the possibility of leading to interesting biochemical applications.40,41 Moreover, once the highly selective chemisorption is integrated with the electronic features of the oligothiophenes, this will open up a path to further advanced device applications. (39) Sentandenkikagaku; Denkikagaku-kyoukai, Ed.; Maruzen: Tokyo, 1994; p 294 (in Japanese). (40) Janata, J. Anal. Chem. 1992, 64, 196R. (41) Biosensors; Turner, A. P. F., Karube, I., Wilson, G. S., Eds.; Oxford University Press: Oxford, 1987; pp 133-548.

Conclusion Structural characterization of the thin films 1a and 2a evaporation-deposited on suitable substrates has been carried out by IR, XPS, and X-ray diffraction methods. As a result, these thin films are found to be deposited without causing chemical alteration or degradation during the course of deposition. Among these films, 1a is characterized by the molecular layered structure where the molecular long axis of 1 is nearly vertical to the substrate plane. The interplay of the π-π interaction between the oligothiophene backbones and the hydrogen bonding between the amino groups is supposed to be responsible for this regular structure. The film 2a, on the other hand, is amorphous, indicating that the chemical modification (with hydrochloric acid) influences the overall morphology of the material. Since the molecular layered structure in 1a is featured by the densely packed array of the amino groups at the film surface, this structure enables the surface modification with suitable reagents through their selective chemisorption. For this purpose, we used pentafluorobenzaldehyde (PFB) as such a reagent. The surface-modified film 1b exhibits an XPS spectroscopic profile pretty different from that of 1a. A new peak due to fluorine (the F 1s line) is clearly seen at 689.2 eV, as expected. Another new peak of the C 1s line observed at 287.9 eV results from the fluorinated aromatic carbons of PFB. Furthermore, the N 1s line is shifted toward the higher energy region relative to that for 1a and assigned to the azomethine nitrogen. The quantitative estimation using the XPS signal ratios of N/C and F/C indicates that the selective chemisorption of PFB takes place satisfactorily. The electrochemical data demonstrate that the immobilization of GOD having a lot larger molecular weight than PFB is well achieved as well. All these results presented in this article show that the oligothiophene compounds that have the chemically active amino groups are evaporation-processed stable. The resulting thin film 1a can be modified on their surface with various chemical reagents. LA9605706